1. Field of the Invention
The present invention relates to an air conditioner for cooling or heating air in a room and a method for controlling an electronic expansion valve of the air conditioner.
2. Description of the Related Art
Generally, an air conditioner is an appliance for cooling or heating an indoor space, such as a residential space, a restaurant, an office room, or etc. The air conditioner comprises a compressor for compressing a refrigerant into a high-temperature and high-pressure gaseous state, a condenser for condensing the refrigerant passing through the compressor into a high-temperature and high-pressure liquid state, an expansion device for decompressing the refrigerant passing through the condenser into a low-temperature and low-pressure liquid state, and an evaporator for evaporating the refrigerant passing through the expansion device into a low-temperature and low-pressure gaseous state. The compressor, the condenser, the expansion device, and the evaporator are connected by a refrigerant pipe. The operation of such an air conditioner is controlled by a microcomputer.
A heat pump-type air conditioner further comprises a direction change valve, such as a 3-way valve or a 4-way valve, adapted to change the flow direction of the refrigerant based on a cooling/heating function, thereby being selectively operated in a cooling or heating mode.
In the cooling mode of the air conditioner, an outdoor heat exchanger functions as the condenser, and an indoor heat exchanger functions as the evaporator. On the other hand, in the heating mode of the air conditioner, the outdoor heat exchanger functions as the evaporator, and the indoor heat exchanger functions as the condenser.
In the cooling mode, the air conditioner allows indoor air to pass through the indoor heat exchanger functioning as the evaporator, thereby discharging cold air to the room. In the heating mode, the air conditioner allows indoor air to pass through the indoor heat exchanger functioning as the condenser, thereby discharging warm air to the room.
A recently developed air conditioner employs an inverter-type compressor which can variably change its refrigerant compression capacity in accordance with a cooling or heating load in a room. Accordingly, it is possible to properly cope with the variation of the cooling or heating load, thereby optimizing the cooling or heating efficiency of the air conditioner.
Generally, a capillary tube or an electronic expansion valve may be used as the expansion device. The electronic expansion valve is mainly used as the expansion device for adjusting the flow rate of the refrigerant so that the cooling or heating capacity of the air conditioner can be variably changed.
Here, an opening degree of the electronic expansion valve is controlled so that a current degree of superheat obtained from a difference between a refrigerant temperature at an inlet of the compressor and a refrigerant temperature at the evaporator coincides with a predetermined target degree of superheat.
Of course, a table, which includes target degrees of superheat in accordance with the refrigerant compression capacities of the compressor, is stored in the microcomputer in advance.
When a user selects one of the cooling and heating modes and sets a desired target temperature, the compressor is operated such that the refrigerant compression capacity of the compressor is variably changed in accordance with a cooling or heating load determined by a difference between the target temperature and a room temperature. A current degree of superheat is measured, and a target degree of superheat is calculated according to the refrigerant compression capacity of the compressor. The opening degree of the electronic expansion device is controlled so that the current degree of superheat coincides with the target degree of superheat, thereby allowing the flow rate of the refrigerant to be controlled. Under this condition, the refrigerant passes through the compressor, the condenser, the electronic expansion valve, and the evaporator, thus allowing the air conditioner to achieve a cooling or heating function.
Recent air conditioners have increased in size, thus requiring a plurality of compressors. In this case, one or more compressors are selectively operated so that the total refrigerant compression capacity of the operating compressors is variably changed. Accordingly, the opening degree of the electronic expansion valve must be controlled so that the flow rate of the refrigerant is variably changed in consideration of the total refrigerant compression capacity.
Here, when the flow rate of the refrigerant, i.e., a refrigerant circulation capacity, is smaller than the refrigerant compression capacity, the compressors are overheated. When the flow rate of the refrigerant is larger than the refrigerant compression capacity, the refrigerant in a liquid state is introduced into the compressors, thus causing failure of the compressors and a limit of improving the cooling or heating efficiency of the air conditioner.
In case that an outdoor temperature is high in the cooling mode of the air conditioner, heat is transmitted to the outdoor heat exchanger serving as a condenser in outdoor air, thereby increasing the cooling load. In case that the outdoor temperature is high in the heating mode of the air conditioner, heat is transmitted to the outdoor heat exchanger serving as an evaporator in outdoor air, and the temperature of the refrigerant passed through the outdoor heat exchanger and the temperature of the refrigerant introduced into the compressors after it passes through the outdoor heat exchanger are comparatively increased, thereby causing the compressors to overheat. Thus, the opening degree of the electronic expansion valve must be controlled in consideration of the outdoor temperature.
Accordingly, in order to control the opening degree of the electronic expansion valve in consideration of the total refrigerant compression capacity in the air conditioner comprising the plural compressors, the current degree of superheat must be measured and the target degree of superheat must be set in consideration of the total refrigerant compression capacity and the outdoor temperature. Further, the opening degree of the electronic expansion valve must be controlled so that the current degree of superheat coincides with the target degree of superheat.